back in washington, president obama also observed a moment of silence at the white house as did the u.s. senate. but tempers flared earlier at a senate hearing. iowa senator chuck grassley was one of several republicans who've said the bombings, allegedly by two chechen immigrants, raise questions about immigration reform. >> i think we're taking advantage of an opportunity whe once in 25 years we deal with immigration to make sure that every base is covered. >> brown: that sparked a heated exchange between grassley and new york democrat chuck schumer. >> i say that particularly those who are pointing to what happened, the terrible tragedy in boston, as i would say an excuse for not doing a bill or delaying it many months and years. >> i never said that. >> i didn't say you did sir. >> i didn't say anything about... >> i didn't mean you, mr. grassley. >> those remarks were not aimed at anyone on this committee or the three witnesses. there are people out there; you've read it in the newspapers; they've said it. >> brown: there were also questions about the f.b.i.'s investigation last ye